It has been known for a long time that hydrophobic compounds are extractable from finely comminuted dry substance mixtures with nonpolar solvents, e.g., aliphatic hydrocarbons such as hexane, vegetable oils or fatty acid esters of long-chain fatty acids such as stearic acid methyl ester. Use is made thereof, in particular, in the production of hydrophobic substances such as dyes from native materials.
It has also been known that vegetable oils, e.g., from oil seed meals may be extracted with hexane as the extraction agent. Correspondingly, the determination of the residual oil contents of de-oiled seed oil meals by means of lightly volatile hydrocarbons such as hexane in a Soxhlet apparatus proved successful. In this way it is equally possible to determine in a simple way and within a short time the residual cocoa butter contents in commercial dry and de-oiled cocoa powder by means of Soxhlet extraction.
Those versed in the art are aware that such extractions can successfully be performed only if water is carefully separated from the substance mixtures (here oil seed meals and the cocoa) prior to the solvent extraction. As soon as the mixture includes a humidity content exceeding the small water amounts absorptively drawn by the solid from the ambient air, such extractions of hydrophobic compounds with nonpolar solvents are hardly possible. In the case of the addition of water quantities to dry cocoa powder which quantities are still completely absorbed by it without destroying the powder structure, for instance, extraction of the residual cocoa butter content is no longer possible. This applies for instance also for carrots and other substantially fat-free native substances such as red pepper pods, from which dyes not bound to oil, fats or waxes can successfully be extracted with oils or for instance also with hexane only if and when the starting quantity has sufficiently been predried. As commonly known, such drying is not only complicated but causes frequently losses by thermal or oxidative decomposition even when taking respective precautionary measures.
As compared thereto, extractions in the presence of water are possible if more polar components which while hardly being soluble in water are extracted with more polar extraction solvents not miscible with water which however easily solve these components (e.g., esters of short-chain alkyl alcohols with short-chain carboxylic acids). The substances to be extracted as well as the solvents are, as a rule, compounds which in the chemical structure include both polar and nonpolar structural features and where the nonpolar structural features are not very dominant. In these cases, the presence of humidity or water has no extraction preventing effect. Penicillin for instance may rapidly be extracted with a high yield with butyl acetate from aqueous acidic fermentation broth. In principle, it is of course possible to simultaneously extract by means of such an extraction solvent including both polar and nonpolar structural features also a strongly hydrophobic or, in other words, nonpolar component in the presence of water as well. In this case, however, extraction is not selective and more polar components are inevitably extracted as well. This refers particularly to native substances since they always include a broad spectrum of differently polar components (from strongly nonpolar to strongly polar). The above-described prior art describes processes wherein, by means of a nonpolar extraction solvent of the kind referred to and graded in the literature as non-soluble in water, especially strongly hydrophobic components having strongly dominating nonpolar structural features can selectively be extracted.
In practice, however, there exists very frequently the problem of isolation of small quantities of such very hydrophobic compounds from water-containing substance mixtures. In case of isolation of biologically active strongly hydrophobic effective substances from vegetable or animal material, these are metabolites (such as .beta.-carotin) of living organisms which very often are sensitive to the influence of heat and air. It is exactly here where the drying measures hitherto required constitute a considerable disadvantage.
In common fat extractions, in particular also for the determination of residual oil contents, devices are employed which operate comparable to the Soxhlet apparatus and which commonly pass the extraction solvent in a continuous process and in large quantities through the raw material. The solvent loaded with the substance to be extracted is then separated by distillation, freeze-drying or other suitable measures from the dissolved substance. In addition, it is possible in principle to mix the extraction solvent with the finely comminuted raw material and to press off or filter off the extraction solvent or to expose the mixture to a centrifugal field after the malaxing process so that a solid phase and a liquid phase of the solvent loaded with the extracted substances is formed. Such procedure has for instance been revealed in the abstract of JP3020397(Database WPI, Week 9110, Derwent Publ. Ltd., AN 91-070562), where for the production of fish oil from comminuted fish, the extraction solvents n- or i-propanol are used and the extraction solvent with the extracted lipids is selectively filtered off or centrifuged off from the solids. U.S. Pat. No. 2,875,061 discloses to subject a water-containing fish mush having a fat content of more than 4%, prior to dehydration, hydrolysis, and deodorization, to a solid-liquid fat extraction with a non water-miscible organic solvent such as benzene, methylene chloride or trichloroethylene. These solvents are somewhat water-soluble, though. In addition, these solvents as known dissolve most of the fats well and act in a way as to extract other accompanying substances as well so that the selective effect mentioned above cannot be achieved. This is in the case of the separation of fish oil from fish less significant since referred to the fish oil the proportion of that having extracted accompanying substances amounts to only from a few parts per thousand to a few percent maximum. In U.S. Pat. No. 3,565,634, fat is extracted in a special apparatus from fish meal under the addition of water and a fat solvent not explicitly defined, wherein in a way known per se the extraction phase obtained in a counter current or counter flow extraction apparatus is freed from the solvent in a screw centrifuge and a subsequent counter current evaporator.
Since it has been known as already mentioned above that for extractions in general and also for fat extractions huge quantities of extraction solvents are required, it is recommended, particular for the production of oils, fats or waxes from respective native oil, fat or wax containing starting substances in a high yield, as an alternative to an extraction process, a purely physical separation process as has for instance been disclosed in DE 195 29 795 A1 (corresponds to co-pending U.S. application Ser. No. 08/793,082, assigned to the assignee of the present invention). In accordance with this process, it is possible to centrifuge off for instance oils as a second liquid phase without employing an extraction solvent just through one single centrifuging step in the presence of water and a water-soluble organic solvent (preferably in the form of an alcohol not acting in the present case as extraction agent). The first liquid phase is formed by the alcoholic water phase lying under the oil phase, which water phase is also clearly separated from the simultaneously-formed solid phase of the developing solid-liquid-liquid three-phase system. The alcohol acts here as a kind of displacing means and avoids the formation of an otherwise inevitable emulsion layer between aqueous phase and oil phase. Together with the water in which it is well soluble the alcohol provides that the three phases, particularly the two liquid phases develop based on their density difference in the centrifugal field with a clean phase boundary. If the density difference between the two liquid phases cannot be easily adjusted alone with the water-soluble solvent, or the alcohol, respectively, then DE 195 29 795 A1 suggests to add non water-soluble light solvents such as hexane or acids and salts which will provide for the required shift of the density conditions. In DE 195 29 795 A1, the process was performed with or without the addition of hexane, particularly in connection with linseed in order to obtain linseed oil. In both cases, yields could be obtained which were substantially in the same range. The added quantities of water soluble alcohol to obtain the density differences in the centrifugal process are small compared to the required quantities of an oil-extracting solvent in an alternative oil solvent-extraction process. In addition, the centrifugal process may be performed with only one centrifugal step rapidly and with a high yield and short malaxing times. In this case, however, oil, fat or wax containing starting materials such as oil fruits are a prerequisite, and in the separated oil phase only the oil-soluble components of the upper floating or supernatant oil phase can be separated which go into the oil during their generation in the biological growth process and hence were already solved in the oil prior to the separation through the centrifugal process. Basically, in such a centrifugal process only such components of a supernatant phase can be separated that are present in a liquid phase.
In contrast, the inventors of the present invention had to solve the problem of finding a most efficient solvent extraction process which makes possible the successful selective extraction of especially strongly hydrophobic compounds which are present in solid form, from substance mixtures by means of nonpolar solvents where the substance mixtures may be water-containing and need not be pre-dried. The above-described disadvantages and limitations of the prior art are avoided by the present invention.